The necessity for providing sewage treatment plants to clean and purify water discharged from homes and factories is well known. The function of a sewage treatment facility is to receive raw sewage (water containing waste material) as discharged from a city sewage system and clean it to ultimately produce purified water. This is accomplished through a series of biological and mechanical processes.
In a typical water treatment facility, the raw sewage is received from the sewage system and passed through a coarse screen to remove large pieces of matter. The sewage is next directed to one or more primary sedimentation tanks or clarifiers. The sewage remains in primary sedimentation for a period of time sufficient to allow the majority of the heavy matter to settle to the bottom of the tank forming sludge. This sludge is removed for "digestion" by microorganisms. The digested sludge is then dried and can be used as compost or fertilizer. The remaining liquid is treated in a second biological system to remove ammonia. The liquid from this treatment is then aerated and passed into final sedimentation water treatment tanks to remove any remaining solid material.
Water treatment tank configurations vary with each treatment facility application. This notwithstanding, most final sedimentation water treatment tanks are circular. This simplifies automatic skimming, churning and/or bottom scraping operations. More specifically, by eliminating inaccessible corners and providing uniform surfaces, a revolving scraper arm or skimmer blade can provide complete and efficient churning and prevent sludge buildup. Water treatment tanks with non-circular configurations are not as common but are also used.
During operation of one type of circular water treatment tank having an inboard launder channel arrangement, water containing sediment enters the tank from influents located in both the central region of the circular tank and the outer perimeter region of the tank. If the launder channel, explained more fully in the next paragraph, does not prevent the water flow between the central region and the outer perimeter region of the tank, then the sediment-containing water can enter the tank from a single influent located anywhere in the tank. In a continuous process, the lighter clean water is effectively decanted from the heavier sediment containing water. More particularly, the clean water is displaced from the tank by the constant flow of sediment-containing water into the tank. The displaced clean water is forced to flow into an inboard launder channel which is disposed about the center of the tank.
The inboard launder channel is configured such that the inner perimeter of the launder channel is located a specified distance radially away from the center of the tank and the outer perimeter of the launder channel is located a greater specified distance away from the center of the tank, but is not contiguous with the outer perimeter of the tank. The resulting configuration permits sediment-containing water entering the tank to displace cleaner water which is forced to flow under the baffle plate and over the weir located at the inner perimeter of the launder channel. The configuration further permits sediment-containing water entering the tank to displace cleaner water which is forced to flow under a baffle plate and over a weir located at the outer perimeter of the launder channel. This system is commonly referred to as an inboard launder channel configuration.
FIG. 5 also shows a non-circular water treatment tank which can also have an inboard launder channel arrangement. In this configuration, sediment-containing water enters the tank and proceeds to displace the clean water into a plurality of parallel launder channels. Each launder channel has both a weir and a scum baffle plate located at each side of the channel.
In the circular tank configuration, the baffle plate and weir, the functions of which will be explained more fully later, are circular in shape when seen in the plan view. As explained previously, they are typically located at the inner and outer perimeters of the launder channel. The displaced clean water ultimately enters the launder channel which directs the water to the next treatment stage where it is chlorinated and further made safe to be discharged into a river or stream.
Presently, algae growth in the launder channel is a serious problem in clarifier tanks. Specifically, as algae builds up on the surfaces of the clarifier tank, particularly on the weirs, it can substantially interfere with the hydraulic flow therethrough. Algae typically adheres to the wet surfaces of the weir and the channel, where it becomes unsightly and odorous. When the launder channel is cleaned, however, the algae is often loosened and causes clogging of the downstream filters.
For many years, the removing of algae from the baffle, weir, spillway and clean water flow channel has been completed primarily by scrubbing the tank structure with brushes manipulated by hand. Because the final treatment tanks are quite large, this is a labor-intensive and tedious process, involving a large expenditure of man-hours. Additionally, the algae removing process must be done frequently, thereby further adding to the cost. The additional time and cost of manually cleaning the baffle, weir and spillway is compounded by the inboard launder configuration, since the inboard launder configuration typically utilizes two scum baffles, weirs and spillways, as opposed to a single baffle, weir and spillway in a conventional configuration. This results in roughly twice the surface area which needs to be kept free of or cleaned of algae build-up. Furthermore, since the inboard launder channel is centrally located in the tank, rather than along the tank's outer perimeter, it is difficult for a worker to manually reach the baffles, weirs and spillways in order to clean them by hand, forcing additional expediture of resources to insure the safety for the workers employed for the task.
More recent proposals directed at the problem of algae growth have utilized mechanical brushes to automate the cleaning process. One such device is disclosed in U.S. Pat. No. 4,830,748. While this apparatus is somewhat effective in cleaning the baffle, weir, spillway and clean water flow channel of a circular tank, it is somewhat limited in application in that it is adapted to be mounted to a revolving skimmer blade. Accordingly, it can only be effectively utilized with circular water treatment tanks, wherein the launder channel is disposed about the outer perimeter of the tank, incorporating such a blade. The cost of obtaining, installing, and maintaining such an automated system, though perhaps preferable to the alternative of periodic manual scrubbing, is also quite high. An inboard launder configuration is even less suitable for this type of mechanical brush operation, since the inner and outer baffles, weirs and spillways would require, in the circular tank configuration, two mechanical brush systems--one which brushed the inner baffle, weir and spillway from the center of the tank, and a second which rotated around the outer perimeter of the tank in order to reach the baffle, weir and spillway located at the outer perimeter of the launder channel. The non-circular tank configuration, due to its irregular shape as shown in FIG. 5, is also unsuitable for a mechanical brush system.
Other waste water facilities have utilized chlorine and other chemicals in sufficient concentrations to kill the algae. However, a large number of waste water facilities utilize a denitrification process that precludes the use of such chemical additives. As such, a need is recognized for a proactive system that inhibits the growth of algae in the inboard launder channel of a clarifier tank.